This invention relates generally to a reproducing machine, and more particularly concerns the developer material employed therein to render a latent image visible.
Though there are various types of reproducing machines, the most commonly available are electrostatographic printing machines. In an electrostatographic printing machine, a latent image is recorded on a surface and rendered visible with particles. These particles may be transferred to a sheet of support material, in image configuration, or remain on the recording surface. In either of the foregoing cases, the particles are permanently affixed to the sheet of support material or recording surface. Thus, a copy of an original document is formed.
Electrostatographic printing includes both electrophotographic and eletrographic printing. Electrophotographic printing employs a light image of the original document to dissipate a charged photoconductive surface. This results in a latent image of the original document being recorded on the photoconductive surface. The latent image is subsequently developed with particles. Electrographic printing does not employ a photoconductive member or a light image to create a latent image of the original document. In general, both of the foregoing processes employ particles to develop the latent image. These particles are subsequently permanently affixed to the copy sheet by the application of heat and/or pressure thereto.
A typical developer material comprises a finely divided colored material called "toner" adhering triboelectrically to a slightly more coarsely divided material called a "carrier". This two-component developer material contacts the latent image. The toner particles are attracted from the carrier granules to the latent image rendering it visible. Generally, the carrier granules are made from a ferro-magnetic material while the toner particles are made from a thermoplastic material. Typically, the toner particles used in a two-component developer mixture of this type have resistivities ranging from about 10.sup.14 to about 10.sup.17 ohm-cm.
Various methods have been developed for applying the developer material to the latent image. For example, the developer material may be cascaded over the latent image and the toner particles attracted from the carrier granules thereto. Other apparatus employed to develop the latent image include the use of polar members or magnetic field producing devices forming brush-like tufts extending outwardly therefrom and contacting the photoconductve surface.
With the advent of single component developer materials, i.e. conductive particles, carrier granules are no longer required. U.S. Pat. No. 2,846,333 issued to Wilson in 1956 describes a typical single component developer material. In general, the charged particles employed as a single component developer material have low resistivities which ranges from about 10.sup.4 to about 10.sup.9 ohm-cm. These particles are developed on the latent image. However, when the particles are transferred to the copy sheet repulsion occurs. Transfer is optimized by employing particles having high resistivities. Contrawise, development is optimized by utilizing particles having low resistivity or good conductivity. Thus, the system is faced with two contradictory requirements, i.e. toner particles having low resistivity for optimum development and high resistivity for optimum transfer.
Hereinbefore, various types of developer materials have been developed. For example, U.S. Pat. No. 3,639,245 issued to Nelson in 1972 discloses particles having a highly resistive interior core and a highly conductive surface or shell. This patent also discloses particles made from a thermoplastic material having conductive particles embedded therein. U.S. Pat. No. 3,816,840 issued to Kotz in 1974 also describes particles having an insulating core with a conductive exterior surface. In both of the foregoing patents, the particles are described as being magnetic. U.S. Pat. No. 3,345,294 issued to Cooper in 1967 discloses particles made from a polyamide resin having magnetic particles dispersed there. An electrostatic ink powder made from thermoplastic resin having a coating of carbon adhering thereto is described in U.S. Pat. No. 3,196,032 issued to Seymour in 1965. Other patents describing a developer comprising particles dispersed in a liquid are U.S. Pat. Nos. 3,155,513; 3,241,998; 3,244,633; 3,406,062; and 3,438,904. U.S. Pat. Nos. 3,607,363 and 3,775,103 disclose photoconductive particles dispersed in a resin or mixed with a non-conductive powder. Metal carriers coated with an acrylic resin or a polymer are described in U.S. Pat. Nos. 3,900,414 and 3,939,086. Finally, U.S. Pat. No. 3,766,072 discloses a developer comprising insulating and conductive particles; and U.S. Pat. No. 3,901,695 describes a metal carrier blended with infrangible and frangible resins.
Accordingly, it is a primary object of the present invention to improve single component developer materials employed to render latent images visible.